1. Field of the Invention
The present invention relates to improvement of the feed screw mechanism comprising a feed screw shaft of a cantilever type having a free end, a nut screwed on the shaft and a vibration damping member mounted on the screw shaft.
2. Related Background Art
As the vibration damping means for the cantilever type having a free end feed screw mechanism, there has been conventionally used a ring member or a bearing. For example, the invention utilizing the ring member or the bearing had been already proposed in U.S. Pat. No. 4,671,127. The cited reference discloses regarding that in a case the vibration damping means is a ring member, the inner diameter of the vibration damping member is so determined as to be slightly larger than the outer diameter of the portion of the screw shaft on which the vibration damping member is mounted.
When such a ring member is used as a vibration damping means therein, the clearance between the outer diametric circumferential surface of the mounting portion of the shaft and the inner diametric circumferential surface of the ring member allows the ring member to move therein. This collision between the ring member and the mounting portion of the shaft produces a damping effect against the radial vibration of the feed screw shaft. On the other hand, in a case where the vibration damping means is a bearing, a damping effect is produced by the collision between inner race and balls and also between outer race and balls to damp the radial vibration of the feed screw shaft.
An example of such cantilever type feed screw mechanism according to the prior art will be described in detail with reference to FIG. 15 of the accompanying drawings.
Referring to FIG. 15, the base 1 has an upright part 2 in which one end of a feed screw shaft 4 is rotatably supported by way of a bearing 3. A portion of the one end (support end) of the screw shaft 4 projects from the upright portion 2 of the base. A belt pulley 5 is fixed on the projected portion of the screw shaft.
A ball nut 6 is screwed onto the middle portion of the feed screw shaft 4 so that the thread groove formed on the inner circumferential surface of the ball nut 6 cooperates with the ball thread groove 7 formed on the outer circumferential surface of the feed screw shaft by way of balls. The ball nut 6 is fixed to a slide table 8 of a moving member.
The free end of the feed screw shaft 4 is provided with a mounting portion 9 having a smaller diameter. A ring member 10 is mounted as vibration damping means thereon.
The ring member 10 is mounted on the mounting portion 9 by making the mounting portion pass through a through hole 11 of the ring member to hold it on the mounting portion 9 by a stop ring 12. The inner diameter of the through hole 11 is so measured as to be slightly larger than the outer diameter of the mounting portion 9, thereby providing some clearance therebetween. In addition, the distance from the stepped portion 13 of the shaft to the stop ring 12 is formed larger than the width of the ring member 10.
In the prior art, the ring member 10 mounted on the free end of the feed screw shaft 4 is allowed to move in the radial direction of a screw feed shaft 4 within the range determined by the clearance between the mounting portion 9 and the ring member 10. Therefore, when vibration is applied to the feed screw shaft, a damping effect is produced by the collision between the mounting portion 9 and the ring member 10. By the damping effect, the radial vibration of the feed screw shaft 4 and the noise generation by such vibration are restrained.
However, the above prior art mechanism involves the problem that lubricant such as oil or grease inevitably flowing along the outer circumferential surface of the feed screw shaft 4 penetrates into the clearance formed between the inner surface of the through hole 11 of the ring member 10 and the outer surface of the mounting portion 9. When such penetration of lubricant occurs, the collision between the mounting portion 9 and the ring member 10 brings about unnecessary buffered state by the viscosity of the lubricant.
As the result, the damping effect by the collision is reduced to a great extent as compared to the damping effect obtained when no lubricant is present in the clearance. Thus, the ability of the ring member for damping the vibration of the feed screw shaft 4 is decreased.
In order to solve the problem, it may be proposed to increase the clearance between the inner surface of the ring member 10 and the mounting portion 9 of the feed screw shaft. However, this solution brings about another problem that the noise generated by the collision is increased and also the damping effect obtained by the collision is reduced.